The present invention relates to the use of quantum dots in light-emitting devices. The invention further relates to light-emitting devices that emit light of a tailored spectrum of frequencies. In particular, the invention relates to a light-emitting device, wherein the device is a light-emitting diode.
Light-emitting devices, in particular, light-emitting diodes (LEDs), are ubiquitous to modern display technology. More than 30 billion chips are produced each year and new applications, such as automobile lights and traffic signals, continue to grow. Conventional devices are made from inorganic compound semiconductors, typically AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue). These devices emit monochromatic light of a frequency corresponding to the band gap of the compound semiconductor used in the device. Thus, conventional LEDs cannot emit white light, or indeed, light of any xe2x80x9cmixedxe2x80x9d color, which is composed of a mixture of frequencies. Further, producing an LED even of a particular desired xe2x80x9cpurexe2x80x9d single-frequency color can be difficult, since excellent control of semiconductor chemistry is required.
Light-emitting devices of mixed colors, and particularly white LEDs, have many potential applications. Consumers would prefer white light in many displays currently having red or green light-emitting devices. White light-emitting devices could be used as light sources with existing color filter technology to produce full color displays. Moreover, the use of white LEDs could lead to lower cost and simpler fabrication than red-green-blue LED technology.
White LEDs are currently made by combining a blue LED with a yellow phosphor to produce white light. However, color control is poor with this technology, since the colors of the LED and the phosphor cannot be varied. This technology also cannot be used to produce light of other mixed colors.
It has been proposed to manufacture white or colored light-emitting devices by combining various derivatives of photoluminescent polymers such as poly(phenylene vinylene) (PPVs). One device that has been proposed involves a PPV coating over a blue GaN LED, where the light from the light-emitting device stimulates emission in the characteristic color of the PPV, so that the observed light is composed of a mixture of the characteristic colors of the device and the PPV. However, the maximum theoretical quantum yield for PPV-based devices is 25%, and the color control is often poor, since organic materials tend to fluoresce in rather wide spectra. Furthermore, PPVs are rather difficult to manufacture reliably, since they are degraded by light, oxygen, and water. Related approaches use blue GaN-based LEDs coated with a thin film of organic dyes, but efficiencies are low (see, for example, Guha et al. (1997) J. Appl. Phys. 82(8):4126-4128; III-Vs Review 10(1):4, 1997).
It has also been proposed to produce light-emitting devices of varying colors by the use of quantum dots (QDs). Mattoussi et al. (1998) J. Appl. Phys. 83:7965-7974; Nakamura et al. (1998) Electronics Lett. 34:2435-2436; Schlamp et al. (1997) J. Appl. Phys. 82:5837-5842; Colvin et al. (1994) Nature 370:354-357. Semiconductor nanocrystallites (i.e., QDs) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of QDs shift to the blue (higher energies) as the size of the QDs gets smaller. It has been found that a CdSe QD, for example, can emit light in any monochromatic color, in which the particular color characteristic of the light emitted is dependent only on the QD""s size.
Currently available light-emitting diodes and related devices that incorporate quantum dots use QDs that have been grown epitaxially on a semiconductor layer. This fabrication technique is most suitable for the production of infrared light-emitting devices, but devices in higher-energy colors have not been achieved by this method. Further, the processing costs of epitaxial growth by currently available methods (molecular beam epitaxy and chemical vapor deposition) are quite high. Colloidal production of QDs is a much more inexpensive process, but QDs produced by this method have generally been found to exhibit low quantum efficiencies, and thus have not previously been considered suitable for incorporation into light-emitting devices.
A few proposals have been made for embedding colloidally produced QDs in an electrically conductive layer in order to take advantage of the electroluminescence of these QDs for a light-emitting device. Mattoussi et al. (1998), supra; Nakamura et al. (1998), supra; Schlamp et al. (1997), supra; Colvin et al. (1994), supra. However, such devices require a transparent, electrically conductive host matrix, which severely limits the available materials for producing devices by this method. Available host matrix materials are often themselves light-emitting, which may limit the achievable colors using this method.
In one aspect, this invention comprises a device, comprising a light source and a population of QDs disposed in a host matrix. The QDs are characterized by a band gap energy smaller than the energy of at least a portion of the light from the light source. The matrix is disposed in a configuration that allows light from the source to pass therethrough. When the QD disposed in the host matrix is irradiated by light from the source, that light causes the QDs to photoluminesce secondary light. The color of the secondary light is a function of the size, size distribution and composition of the QDs.
In one embodiment of this aspect, the QDs comprise a core of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, an alloy thereof, or a mixture thereof, and are, optionally, overcoated with a shell material comprising ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaAs, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs, AlN, AlP, AlSb, an alloy thereof, or a mixture thereof. Preferably, the band gap energy of the overcoating is greater than that of the core. The core or core-shell QD may be further coated with a material having an affinity for the host matrix. The host matrix may be any polymer, such as polyacrylate, polystyrene, polyimide, polyacrylamide, polyethylene, polyvinyl, poly-diacetylene, polyphenylene-vinylene, polypeptide, polysaccharide, polysulfone, polypyrrole, polyimidazole, polythiophene, polyether, epoxies, silica glass, silica gel, siloxane, polyphosphate, hydrogel, agarose, cellulose, and the like. The primary light source may be a light-emitting diode, a laser, an arc lamp or a black-body light source. The color of the device is determined by the size, size distribution and composition of the QDs. The size distribution may be a random, gradient, monomodal or multimodal and may exhibit one or more narrow peaks. The QDs, for example, may be selected to have no more than a 10% rms deviation in the diameter of the QDs. The light may be of a pure color, or a mixed color, including white.
In a related aspect, the invention comprises a method of producing a device as described above. In this method, a population of QDs is provided, and these QDs are dispersed in a host matrix. A light source is then provided to illuminate the QDs, thereby causing them to photoluminesce light of a color characteristic of their size, size distribution and composition. The QDs may be colloidally produced (i.e., by precipitation and/or growth from solution), and may comprise a core of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, an alloy thereof, or a mixture thereof. The QDs are, optionally, overcoated with a shell material comprising ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, an alloy thereof, or a mixture thereof. The host matrix may be any material in which QDs may be dispersed in a configuration in which they may be illuminated by the primary light source. Some examples of host matrix materials include polyacrylate, polystyrene, polyimide, polyacrylamide, polyethylene, polyvinyl, poly-diacetylene, polyphenylene-vinylene, polypeptide, polysaccharide, polysulfone, polypyrrole, polyimidazole, polythiophene, polyether, epoxies, silica glass, silica gel, siloxane, polyphosphate, hydrogel, agarose, cellulose, and the like. Any light source capable of causing the QDs to photoluminesce may be used; some examples are light-emitting diodes, lasers, arc lamps and black-body light sources.
It may be desirable to tailor the size distribution of the QDs of a particular core composition to tailor the color of light which is produced by the device. In one embodiment, referred to herein as a xe2x80x9cmonodisperse size distribution,xe2x80x9d the QDs exhibit no more than a 10% rms deviation in diameter. The light may be of a pure color using a monodisperse size distribution of QDs or of a mixed color using a polydisperse size distribution of QDs, including white.
In a further aspect, the invention comprises a QD composition, in which QDs are disposed in a host matrix. The QDs are, optionally, coated with a material having an affinity for the host matrix. When illuminated by a source of light of a higher energy than the band gap energy of the QDs, the QDs photoluminesce in a color characteristic of their size, size distribution and composition.
In one embodiment, the QDs comprise a core of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, an alloy thereof, or a mixture thereof, and are, optionally overcoated with a shell material comprising ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaAs, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs, AlN, AlP, AlSb, an alloy thereof, or a mixture thereof. The host matrix may be a polymer such as polyacrylate, polystyrene, polyimide, polyacrylamide, polyethylene, polyvinyl, poly-diacetylene, polyphenylene-vinylene, polypeptide, polysaccharide, polysulfone, polypyrrole, polyimidazole, polythiophene, polyether, epoxies, silica glass, silica gel, siloxane, polyphosphate, hydrogel, agarose, cellulose, and the like. In one embodiment, the QDs are coated with a monomer related to a polymer component of the host matrix. The QDs may be selected to have a size distribution exhibiting an rms deviation in diameter of less than 10%; this embodiment will cause the QDs to photoluminesce in a pure color.
A related aspect of the invention comprises a prepolymer composition comprising a liquid or semisolid precursor material, with a population of QDs disposed therein. The composition is capable of being reacted, for example by polymerization, to form a solid, transparent or translucent host matrix, i.e., a host matrix that allows light to pass therethrough. Optionally, the QDs are coated with a material having an affinity for the precursor material or with a prepolymeric material. For example, if the prepolymer composition forms a polyacrylate upon polymerization, the QD can be coated with an acrylate monomer which, optionally, allows the QD to become incorporated into the backbone structure of the polymer. The precursor material may be a monomer, which can be reacted to form a polymer. The QDs may comprise a core of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, an alloy thereof, or a mixture thereof, and are, optionally, overcoated with a shell material comprising ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaAs, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs, AlN, AlP, AlSb, an alloy thereof, or a mixture thereof. The QDs may be selected to have a size distribution having an rms deviation in diameter of less than 10%.
In yet another aspect, the invention comprises a method of producing light of a selected color. The method comprises the steps of providing a population of QDs disposed in a host matrix, and irradiating the QDs in the host matrix with a source of light having an energy higher than the band gap energy of a QD in the host matrix such that the QDs are caused to photoluminesce. The QDs may comprise a core of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, an alloy thereof, or a mixture thereof, and are, optionally overcoated with shell material comprising ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaAs, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs, AlN, AlP, AlSb, an alloy thereof, or a mixture thereof. The host matrix may comprise polymers such as polyacrylate, polystyrene, polyimide, polyacrylamide, polyethylene, polyvinyl, poly-diacetylene, polyphenylene-vinylene, polypeptide, polysaccharide, polysulfone, polypyrrole, polyimidazole, polythiophene, polyether, epoxies, silica glass, silica gel, siloxane, polyphosphate, hydrogel, agarose, cellulose, and the like.
The host matrix containing the QDs may be formed by reacting a precursor material having QDs disposed therein (for example by polymerization or physically entrapping). Alternatively, two or more precursor materials may be provided, each having QDs of a different sizes, size distributions and/or compositions disposed therein. These precursors may be mixed and reacted to form a host matrix, or alternatively, they may be layered to form a host matrix having different sizes, size distributions and/or compositions of QDs in different layers.